Inaccurate swimming route recorded on Apple Watch

I swam for an hour. My usual route is between 1 and 1.5 km.

i swam the usual route.

part of the map was dotted with apparently no swimming recorded.

the total distance recorded was 800m.

inhave a photo.

why does this happen? How can I improve

my location when swimming?

[Re-Titled by Moderator]

Original Title: Partial swimming route recorded